//对象转查询字符串
function objToSearchString(obj){
    var str  = ''
    for(var key in obj){
        str += `${key}=${obj[key]}&`;
    }
    return str.slice(0,str.length-1);
}

//封装ajax函数
function ajax( method, url, params , cb ){
    //1.创建请求对象
    var xhr = new XMLHttpRequest()

    //2.配置请求对象
    xhr.open(method, method == 'get' ? url + "?" + objToSearchString(params) : url );

    //3.设置回调函数
    xhr.onreadystatechange = function(){
        if( xhr.readyState == 4 ){
            cb( JSON.parse(xhr.responseText) )
        }
    }

    //4.发送请求
    //先对post请求设置 content-type 请求头
    xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded');
    xhr.send(  method == 'get' ? null : objToSearchString(params) );
}


